California Penal Code section 1203.4 provides:

(a) In any case in which a defendant has fulfilled the conditions of probation for the entire period of probation, or has been discharged prior to the termination of the period of probation, or in any other case in which a court, in its discretion and the interests of justice, determines that a defendant should be granted the relief available under this section, the defendant shall, at any time after the termination of the period of probation, if he or she is not then serving a sentence for any offense, on probation for any offense, or charged with the commission of any offense, be permitted by the court to withdraw his or her plea of guilty or plea of nolo contendere and enter a plea of not guilty; or, if he or she has been convicted after a plea of not guilty, the court shall set aside the verdict of guilty; and, in either case, the court shall thereupon dismiss the accusations or information against the defendant and except as noted below, he or she shall thereafter be released from all penalties and disabilities resulting from the offense of which he or she has been convicted, except as provided in Section 13555 of the Vehicle Code. The probationer shall be informed, in his or her probation papers, of this right and privilege and his or her right, if any, to petition for a certificate of rehabilitation and pardon. The probationer may make the application and change of plea in person or by attorney, or by the probation officer authorized in writing. However, in any subsequent prosecution of the defendant for any other offense, the prior conviction may be pleaded and proved and shall have the same effect as if probation had not been granted or the accusation or information dismissed. The order shall state, and the probationer shall be informed, that the order does not relieve him or her of the obligation to disclose the conviction in response to any direct question contained in any questionnaire or application for public office, for licensure by any state or local agency, or for contracting with the California State Lottery. Dismissal of an accusation or information pursuant to this section does not permit a person to own, possess, or have in his or her custody or control any firearm or prevent his or her conviction under Section 12021. Dismissal of an accusation or information underlying a conviction pursuant to this section does not permit a person prohibited from holding public office as a result of that conviction to hold public office. This subdivision shall apply to all applications for relief under this section which are filed on or after November 23, 1970.

(b) Subdivision (a) of this section does not apply to any misdemeanor that is within the provisions of subdivision (b) of Section 42001 of the Vehicle Code, to any violation of subdivision (c) of Section 286, Section 288, subdivision (c) of Section 288a, Section 288.5, or subdivision (j) of Section 289, any felony conviction pursuant to subdivision (d) of Section 261.5, or to any infraction.

(c) (1) Except as provided in paragraph (2), subdivision (a) does not apply to a person who receives a notice to appear or is otherwise charged with a violation of an offense described in subdivisions (a) to (e), inclusive, of Section 12810 of the Vehicle Code.

(2) If a defendant who was convicted of a violation listed in paragraph (1) petitions the court, the court in its discretion and in the interests of justice, may order the relief provided pursuant to subdivision (a) to that defendant.

(d) A person who petitions for a change of plea or setting aside of a verdict under this section may be required to reimburse the court for the actual costs of services rendered, whether or not the petition is granted and the records are sealed or expunged, at a rate to be determined by the court not to exceed one hundred fifty dollars ($150), and to reimburse the county for the actual costs of services rendered, whether or not the petition is granted and the records are sealed or expunged, at a rate to be determined by the county board of supervisors not to exceed one hundred fifty dollars ($150), and to reimburse any city for the actual costs of services rendered, whether or not the petition is granted and the records are sealed or expunged, at a rate to be determined by the city council not to exceed one hundred fifty dollars ($150). Ability to make this reimbursement shall be determined by the court using the standards set forth in paragraph (2) of subdivision (g) of Section 987.8 and shall not be a prerequisite to a person's eligibility under this section. The court may order reimbursement in any case in which the petitioner appears to have the ability to pay, without undue hardship, all or any portion of the costs for services established pursuant to this subdivision.

(e) Relief shall not be granted under this section unless the prosecuting attorney has been given 15 days' notice of the petition for relief. The probation officer shall notify the prosecuting attorney when a petition is filed, pursuant to this section. It shall be presumed that the prosecuting attorney has received notice if proof of service is filed with the court.

(f) If, after receiving notice pursuant to subdivision (e), the prosecuting attorney fails to appear and object to a petition for dismissal, the prosecuting attorney may not move to set aside or otherwise appeal the grant of that petition.

(g) Notwithstanding the above provisions or any other provision of law, the Governor shall have the right to pardon a person convicted of a violation of subdivision (c) of Section 286, Section 288, subdivision (c) of Section 288a, Section 288.5, or subdivision (j) of Section 289, if there are extraordinary circumstances.

Many people erroneously refer to the granting of a Penal Code section 1203.4 Petition for Dismissal as an "Expungement."  As noted in People v. Frawley (2000) 82 Cal.App.4th 784, the granting of a Penal Code section 1203.4 Petition for Dismissal is NOT a true expungement:

The central "rule" of the statute is that one who obtains the relief provided "shall thereafter be released from all penalties and disabilities resulting from the offense." (§ 1203.4, subd. (a).) If this language stood alone, without qualification, it would almost certainly be understood to effect a true  “expungement,” such that the prior conviction would cease to exist in the eyes of the law, or at least of the criminal law. However, while a number of courts have used forms of the word "expunge" to describe the relief made available by section 1203.4 (e.g., People v. Acuna (2000) 77 Cal.App.4th 1056, 1058, the statute does not in fact produce such a dramatic result. To "expunge" means "to strike out, obliterate ... efface completely: Destroy" (Webster's 9th New Collegiate Dict. (1984) p. 439) or "blot out, erase... wipe out, ..., annihilate, annul ... put an end to" (5 Oxford English Dict. (2d ed. 1989) p. 588). As used in the criminal law, "expungement,” means the " ' " eradication of a record of conviction or adjudication upon the fulfillment of prescribed conditions .... It is not simply the lifting of disabilities attendant upon conviction and a restoration of civil rights .... It is rather a redefinition of status, a process of erasing the legal event of conviction or adjudication and thereby restoring to the regenerative offender his status quo ante." ' (United States v. Fryer (N.D.Ohio 1975) 402 F.Supp. 831, 834, quoting Grough, Expungementof Adjudication Records, 1966 Wash. U. L. Q. 147, 149.)" (People v. Field (1995) 31 Cal.App.4th 1778, 1786.)

Section 1203.4 does not, properly speaking, "expunge" the prior conviction. The statute does not purport to render the conviction a legal nullity. Instead it provides that, except as elsewhere stated, the defendant is "released from all penalties and disabilities resulting from the offense." The limitations on this relief are numerous and substantial, including other statutes declaring that an order under section 1203.4 is ineffectual to avoid specified consequences of a prior conviction. (E.g., Veh. Code, § 13555 [relief under § 1203.4 "does not affect any revocation or suspension of the privilege of the person convicted to drive a motor vehicle under this chapter"]; Bus. & Prof. Code, §§ 490 [licensing boards can suspend or revoke professional license based on qualifying conviction "irrespective of a subsequent order under the provisions of Section 1203.4 of the Penal Code"], 6102, subd. (c) [summary disbarment of attorney for qualifying conviction "irrespective of any subsequent order under Section 1203.4 of the Penal Code or similar statutory provision"] 2236.1, subd. (d) [same, suspension of medical license]; Ed. Code, §§ 44008, 44009 [ineffectuality of relief under § 1203.4 to affect certain provisions concerning employment in elementary or secondary schools]; cf. Lab. Code, § 26 [no license under Labor Code shall be denied solely due to conviction which has been dismissed under § 1203.4]; see generally Adams v. County of Sacramento (1991) 235 Cal.App.3d 872, 880.) Furthermore, by the statute's own terms, an order under section 1203.4 "does not relieve" the ex-offender of "the obligation to disclose the conviction in response to any direct question contained in any questionnaire or application for public office [or] for licensure by any state or local agency ...." (§ 1203.4, subd. (a).)

Indeed, section 1203.4 contains a sweeping limitation on the relief it offers, stating that "in any subsequent prosecution of the defendant for any other offense, the prior conviction may be pleaded and proved and shall have the same effect as if probation had not been granted or the accusation or information dismissed." This provision alone precludes any notion that the term “expungement” accurately describes the relief allowed by the statute.